Vavilov, Centers of Origin, Spread of Crops

Nicolai Vavilov
(1887-1943) was a Russian scientist who headed the Lenin All-Union
Academy of Agricultural Sciences (later named the Vavilov All-Union
Institute of Plant Industry in his honor) in St. Petersburg (Leningrad)
from 1920 to 1940. He established 400 research institutes that employed
up to 20,000 people. He planned to collect all of the useful germplasm
that had potential in the Soviet Union, to classify it, and to use
it in a national plant breeding effort. He and his colleagues conducted
extensive germplasm explorations and collections in many parts of
the world. The Vavilov Institute remains an important resource for
germplasm maintenance, access, and utilization.

In 1926 he published "Studies on the Origin of Cultivated Plants"
which described his theories on the origins of crops. Vavilov concluded
that each crop has a characteristic primary center of diversity which is
also its center of origin. Eight areas were recognized and suggested as
centers from which all of our major crops were domesticated. Later, he modified
his theory to include "secondary centers
of diversity" for some crops.

In recent years, the diversity of DNA fractions and other approaches
have been used to study diversity of crop species. In general, these
studies
have not confirmed Vavilov's theory that the centers of origin are the
areas of greatest diversity. They have identified centers of diversity,
but these are often not the centers of origin. For some crops there is
little connection between the source of the wild ancestors, areas of
domestication,
and the areas of evolutionary diversification. Species may have originated
in one geographic area, but were domesticated in a different region.
Some
crops do not even have centers of diversity.

In 1971, Jack Harlan described his own views on the origins of agriculture.
He proposed three independent systems, each with a center and a "noncenter"
(larger, diffuse areas where domestication is thought to have occured).

Near East + Africa

China + S. E. Asia

Mesoamerica + S. America

Evidence since that time suggests that these centers are also more diffuse
than he had envisioned. After the initial phases of evolution, the species
spread out over large, ill-defined areas. This is probably due to crops
traveling with man and evolving along the way. Regional and/or multiple
areas of origin may be better models than the idea of a unique, localized
origin for many crops.

Nonetheless, we can make some statements about the probable geographic
origin of many crops:

Biomes and Ecoregions

Biome: A major regional
terrestrial community, or grouping, with its own type of climate, vegetation,
and animal life. Biomes are not sharply separated, but merge gradually
into one another over what is called an ecotone. A biome embraces the
idea of community, interactions among vegetation, animal populations,
and soil types within a regional climate.

Climatic conditions, such as rainfall and temperature, are the major
determinants of the type of biome that develops in an area. Deserts develop
in areas with low annual precipitation. Slightly higher rainfall will
favor a grassland, and high yearly precipitation will favor the development
of a forest. Altitude is another important factor determining the distribution
of biomes. Cool temperatures at high altitudes may favor a type of plant
community normally
encountered
at higher latitudes.

Harlan's new theory (1992): Certain biomes or vegetative types may have
been more conducive to crop domestication than others. The Mediterranean
woodlands and tropical savannas were ideal for domestication because they
both have long dry seasons, which generate annuals.

Description of major biomes

Biomes are classified in various ways. We will discuss eight biomes
in this section, but as you can see from the map below, additional types
can easily be distinguished.

Tropical highland

Ecoregions of the world

National Geographic presents 867 ecoregions, distinguished by shared ecological
features, climate, plant and animal communities. Commonalities in ecological
features or environmental constraints would represent good opportunities
for successful species or varietal introductions.

1) To define target breeding environments (e.g., the CIMMYT maize program
megaenvironment concept)
2) For strategic introduction of germplasm (e.g., the joint CIAT/IITA
program for introduction of Latin American cassava germplasm into Africa)

Agroecological production zones for the Pacific Northwest were defined
by Douglas et al. (1990) based on annual precipitation, soil depth, and
growing degree days. The zones were proposed to facilitate communication
and adoption of appropriate farming technology throughout the region.
For plant breeders in the region, they also serve as a basis for ‘design’
of varieties better adapted and able to ‘exploit’ environmental
variations; i.e., breeding for agroecological adaptation.

Environmental Factors that Affect
Plant Growth

In order to discuss the ways that plants have evolved to be better adapted
to their environments, it is necessary to have a basic understanding of
plant growth requirements. This is a very broad topic, so we will only
outline the important factors and provide links for further information.

Crop Adaptation

In this section we will discuss several mechanisms plants have developed
that make them adapted to particular environments.

Types of Photosynthesis

Photosynthesis is the joining
together of CO2 (carbon dioxide) with H2O (water)
to make CH2O (sugar) and O2 (oxygen), using the
sun's energy. The sugar contains the stored energy and serves as the raw
material from which other compounds are made.

Basic photosynthetic pathway:

6CO2 + 12 H2O --> C6H12O6
+ 6O2 + 6H2O

Energy to carry out the reaction comes from light absorbed by chlorophyll,
stored as ATP and NADH.

There are three types of photosynthesis: C3, C4, and CAM.
The type of photosynthesis utilized by a species influences its adaptation
to different environments.

Respiration is the opposite
of photosynthesis -- the stored energy in the sugar is released in the
presence of oxygen, and this reaction releases the CO2 and
H2O originally joined together by the sun's energy.

Photorespiration

Under high light and high heat, the enzyme (RUBISCO)
that grabs carbon dioxide for photosynthesis may grab oxygen instead,
causing respiration to occur instead of photosynthesis, thus reducing
the production of sugars from photosynthesis.

During photorespiration, O2 + Rubp (ribulosebisphosphate,
a 5-carbon compound) are catalyzed by RUBISCO to produce one molecule
of 3-PGA (3-phosphoglycerate, a 3-carbon organic acid) and one molecule
of phosphoglycolate.

RUBISCO

O2 + Rubp

-------->

3-PGA + phosphoglycolate

Photorespiration can reduce photosynthetic efficiency by 30%. Furthermore,
the phosphoglycolate is toxic and must be broken down by the plant.
Higher
levels of CO2, or lower levels of O2, will increase
photosynthesis by decreasing photorespiration.

C3 Photosynthesis

Most plants use C3 photosynthesis. It is called the C3 pathway because
CO2 is first incorporated into a 3-carbon compound. CO2
and ribulosebisphosphate are combined by RUBISCO, resulting in the production
of two molecules of the 3-carbon organic acid 3-PGA.

RUBISCO

CO2 + Rubp

-------->

2 3-PGA

Photosynthesis takes place throughout the leaf. Stomata are open during
the day.

Photorespiration may occur in C3 plants during light fixation of CO2.

Adaptive Value:
C3 plants are more efficient than C4 and CAM plants under cool and moist
conditions and under normal light because they have less machinery (fewer
enzymes and no specialized anatomy).

C4 Photosynthesis

C4 plants can photosynthesize faster under high heat and light conditions
than C3 plants because they use an extra biochemical pathway and special
leaf anatomy to reduce photorespiration.

The leaves of C4 plants have Kranz anatomy. The
xylem and phloem of these leaves are surrounded by thick walled parenchyma
cells called bundle sheath cells where most of the photosynthesis takes
place.

Stomata open in the morning. CO2 is first combined with phosphoenolpyruvic
acid (PEP) in mesophyll cells by phosphoenolpyruvate carboxylase (PEPCase).
This allows CO2 to be taken into the plant very quickly. The
4-carbon compound oxaloacetic acid is produced, and then converted to
malic or aspartic acid. These are also 4-carbon compounds, hence the name
C4 photosynthesis. The malic or aspartic acid is then moved through plasmodesmata
(at the expense of ATP) into the bundle sheath cells.

In the bundle sheath cells, the malic or aspartic acid is broken into
CO2 and PEP. The CO2 is "delivered" to
the RUBISCO enzyme for photosynthesis. This system allows the plant to
maintain a high concentration of CO2 in the bundle sheath cells
for photosynthesis. The higher concentration of CO2 prevents
photorespiration and allows the plant to close its stomata during the
hot hours of the day.

RUBISCO

CO2 + PEP

-------->

oxaloacetic acid

----->

malic or aspartic acid

----->

CO2 + PEP

mesophyll cells

bundle sheath cells

Adaptive Value: C4 plants
photosynthesize faster than C3 plants under high light intensity and high
temperatures. C4 plants do not have a photorespiration pathway, increasing
photosynthetic efficiency. Water use efficiency of C4 plants is high because
PEP Carboxylase brings in CO2 faster, so the plant does not
need to keep stomata open as much (less water lost by transpiration) to
have sufficient CO2 for photosynthesis. The C4 pathway is more
expensive energetically than normal photosynthesis, but not as expensive
as photorespiration.

C4 plants include several thousand species in at least 19 plant families.
Examples include corn, sorghum, and many of our summer annual plants.

CAM photosynthesis

CAM stands for Crassulacean Acid Metabolism.
Stomata open at night (when evaporation rates are usually lower) and are
usually closed during the day. The CO2 is converted to an acid
and stored during the night. During the day, the acid is broken down and
the CO2 is released to RUBISCO for photosynthesis.

Adaptive
Value: Better water use efficiency than C3 plants under
arid conditions because stomata are open at night when transpiration rates
are lower (no sunlight, lower temperatures, lower wind speeds, etc.).

When conditions are extremely arid, CAM plants can CAM-idle. They leave
their stomata closed night and day. Oxygen given off in photosynthesis
is used for respiration and CO2 given off in respiration is
used for photosynthesis. CAM-idling allows the plant to survive dry spells,
and to recover very quickly when water is available again. CAM plants
include many succulents such as cactuses and agaves and also some orchids
and bromeliads.

The Table below summarizes the features and effects of photosynthetic
pathway on plant adapatation to different environments, growth and productivity.

pineapple, prickly
pear cactus, many orchids, sisal and other agaves, other cactus,
etc.

*The ratio kg water transpired per
kg dry weight produced (low values indicate high water use efficiency)
**Light saturation of a single leaf is indicated by failure of the
CO2 assimilation rate to increase with an increase in light
intensity

Photoperiod

Many angiosperms will flower at about the same time every year, regardless
of when they are planted. This is a response to daylength that promotes
cross-pollination and ensures that plant development is well synchronized
with the length of the growing season. Short-day plants flower only after
exposure to short days, long-day plants flower only after exposure to
long days, and day-neutral plants show no response to daylength.

Some plants can only be grown at certain latitudes due to photoperiodism.
Spinach is a long-day plant, and will never flower if it is grown in the
tropics. Maize is a short-day plant - if a tropical variety is grown at
northern latitudes it will grow very tall and may never flower. Selection
for the day-neutral characteristic has permitted many crops, such as maize
and soybeans, to be grown over much wider geographic areas.

Phytochrome is the plant compound that is responsible for the photoperiod
response. The PR form absorbs red light during the day and
is converted to PFR . The PFR form can absorb far
red light, and will spontaneously convert back to the PR form
in the night. Consequently, it is the length of the night period that
is really important in determining photoperiod response. Short-day plants
should really be called long-night plants and long-day plants should be
called short-night plants.

Vernalization

Vernalization in crops is the acceleration of flowering in response to
a long period of cold temperature. Winter crops require a period of exposure
to temperatures between 0 to 12 °C for a period of time from 10 to
60 days from germination to proceed into the reproductive phase. Vernalization
requirements vary greatly among species and cultivars. Vernalization ensures
that plants overwinter vegetatively and flower in the favorable conditions
of spring. The majority of crops grown in northern Europe and Canada (wheat,
barley, oilseed rape and sugarbeet) have been bred with a strong vernalization
requirement to extend geographical range or prevent bolting.